Flexographic printing plate process

ABSTRACT

Negative-acting photohardenable compositions are useful in flexographic printing plate manufacture without the use of solvent development. Unhardened composition is softened and absorbed by an absorbent sheet. Absorbed composition can be removed from the sheet and recycled.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to a method of using radiation curablepolyurethane elastomeric compositions and a method of makingflexographic printing plates produced with those compositions. Theplates are developable in a solventless absorption processing method.This invention describes compositions which are resistant to swellingwhen immersed in water or solvent based flexographic inks, whichresistance property is critical to the use of these compositions in theform of flexographic printing plates. 2. Background of the Art

Flexography is a term that broadly applies to a printing format using aflexible substrate bearing an elastomeric or rubbery relief printingsurface.

The first flexographic printing plates were produced from natural orsynthetic rubber compositions which were cured chemically under heat andpressure in a mold utilizing conventional rubber curatives such asmercapto compounds (Flexography: Principles and Practices, 3rd Edition,Flexographic Technical Association, p 158-162). More recently,photopolymer elastomeric compositions (elastomer containing compositionscurable upon exposure to actinic radiation) useful to produce reliefprinting plates have been described. For example, U.S. Pat. Nos.4,369,246 and 4,423,135 describe solvent-insoluble, elastomeric printingrelief plates which are formed by applying to a sheet support a layer ofa photosensitive composition comprising (1) at least 30 weight % of asolvent-soluble co-polymer containing at least 2 thermoplastic,non-elastomeric blocks of glass transition temperature above 25° C. andaverage molecular weight 2000-100000 and between these blocks, anelastomeric block copolymer having a glass transition temperature below10° C. and average molecular weight 25,000-1,000,000; (2) at least 1weight % of an addition polymerizable compound containing at least oneterminal ethylenic group; and (3) a polymerization initiator at a drythickness of 0.005-0.250 inch. A flexible polymer film and flexiblecover sheet are laminated to the composition layer. The plate is formedby stripping off the cover sheet, imagewise exposing the layer toactinic radiation through the film, and removing the film and unexposedareas of the layer by solvent washing. Solvents such asperchloroethylene (1,1,1 trichloroethylene) either alone or incombination with alcohols such as n-butanol are utlized. Likewise, EPPat. 261,910 describes a further example of a water-developableflexographic relief printing plate comprised of (1) monomers andpolymers of acrylic acid esters and (2) a ketonephotopolymerizing/photocrosslinking agent, which are coated on a supportsheet. Following imagewise exposure (to promote crosslinking), therelief areas of the plate are produced by washout with an aqueousdeveloper. After washout, all of the flexographic platemakingcompositions and methods described heretofore require drying of theplate for extended periods (1 to 24 hours) to remove entrained developersolution and then are subjected to a post-finishing process (chemical orphotochemical) to reduce surface tack of the plate before use on aprinting press. In addition to the extended time periods required toproduce flexographic printing plates by the aforementioned technologies(by reason of the multiplicity of processing steps required in series),these technologies also produce potentially toxic by-product wastes inthe development procedure. In the case of the solvent-washouttechnology, organic solvent waste is generated which is potentiallytoxic in the form of both the solvent and the addition polymerizablecompound 2) which contains at least one terminal ethylenic group.Likewise, the aqueous washout plate technologies also produce acontaminated waste water by-product stream which may contain similaraddition polymerizable compounds that may have cytotoxic effects.

Many different types of monomers and cross-linkable resins are known inthe polymer art, their properties can be adjusted as taught in the artto provide rigidity, flexibility, or other properties. Particularly goodmaterials related to the compositions of the present invention are shownin U.S. Pat. Nos. 4,578,504; 4,638,040; and 4,786,657.

Various types of curable polyurethane elastomeric compositions areknown. The term "elastomer" or "elastomeric" is used to refer to rubbersor polymers that have resiliency properties similar to those of rubber.In particular, the term elastomer reflects the property of the materialthat it can undergo a substantial elongation and then return to itsoriginal dimensions upon release of the stress elongating the elastomer.In all cases an elastomer must be able to undergo at least 10%elongation (at a thickness of 0.5 mm) and return to its originaldimensions after being held at that elongation for 2 seconds and afterbeing allowed 1 minute relaxation time. More typically an elastomer canundergo 25% elongation without exceeding its elastic limit. In somecases elastomers can undergo elongation to as much as 300% or more ofits original dimensions without tearing or exceeding the elastic limitof the composition. Elastomers are typically defined to reflect thiselasticity as in ASTM Designation D883-866 as a macromolecular materialthat at room temperature returns rapidly to approximately its initialdimensions and shape after substantial deformation by a weak stress andrelease of the stress. ASTM Designation D412-87 can be an appropriateprocedure for testing rubber properties in tension to evaluateelastomeric properties. Generally, such compositions include relativelyhigh molecular weight compounds which, upon curing, form an integratednetwork or structure. The curing may be by a variety of means,including: through the use of chemical curing agents, catalysts, and/orirradiation. The final physical properties of the cured material are afunction of a variety of factors, most notably: number and weightaverage polymer molecular weights; the melting or softening point of thereinforcing domains (hard segment) of the elastomer (which, for example,can be determined according to ASTM Designation D1238-86); the percentby weight of the elastomer composition which comprises the hard segmentdomains; the structure of the toughening or soft segment (low Tg)portion of the elastomer composition; the cross-link density (averagemolecular weight between crosslinks); and the nature and levels ofadditives or adjuvants, etc. The term "cured", as used herein, meanscross-linked or chemically transformed to a thermoset (non-melting) orrelatively insoluble condition.

BRIEF DESCRIPTION OF THE INVENTION

The present invention relates to a process for producing a flexographicprinting plate by providing a radiation hardenable composition(radiation polymerizable, radiation curable, or radiationcross-linkable) as a layer on a flexible substrate, imagewiseirradiating said composition to harden the composition in irradiatedareas, contacting said imagewise irradiated layer with an absorbentlayer which can absorb uniradiated composition when it has been heatedbetween 40° and 200° C., heating said composition layer so that it is ata temperature between 40° and 200° C. while in contact with saidabsorbent layer, said temperature being sufficiently high to enable saidcomposition in unirradiated areas to flow into said absorbent layer,allowing at least 75% of said composition in unirradiated areas incontact with said absorbent layer to be absorbed by said absorbentlayer, and removing said absorbent layer and said at least 75% ofcomposition from unirradiated areas from said flexible substrate. Theprocess can also utilize a predevelopment step that can improve theanchoring of the irradiated composition to the flexible substrate byfirst developing a `floor` on the substrate. The imageable compositionon a substrate which transmits ionizing radiation (e.g., e-beams, shortwavelength U.V. radiation, etc.) is first generally exposed through thesubstrate to generate a floor of polymerized composition. This floor isnot removed from the substrate during development and is not to beconsidered in the determination of the at least 75% of compositionremoved from non-irradiated areas. It is desirable that the elevatedtemperatures used to cause the unirradiated composition to flow (reduceits viscosity, or exceed its softening temperature, e.g., see ASTMDesignation D 1238-86) into and be absorbed by the absorbent layer(without the necessity of reduced air pressure behind the layer) shouldnot distort the flexible substrate or the hardened composition by morethan 2% in any surface dimension. The actual temperatures will vary withthe specific substrate and composition used. Preferably at least 80% ofthe unirradiated composition is removed from the areas heated in contactwith the absorbent layer. More preferably at least 90 or at least 95% isremoved. The hardening or curing step of the process (by irradiation)can also act to increase the adhesion of the composition to thesubstrate. This can be by direct adhesion of the curing composition tothe substrate (either chemical adhesion by reaction or ophysicaladherence to surface structure on the flexible layer) or by adhesion(usually by chemical reaction) to a primer layer on the substrate. Theprimer layer may be photosensitive or photoinsensitive to aid in thisadherence effect.

The present invention can be practiced on some commercially availableflexographic printing plates (e.g., DuPont PLS) but is particularlyuseful with specially designed low temperature melting, high melt index,radiation curable, elastomeric polyurethane compositons useful inproducing flexographic printing plates for use in graphic arts printingapplications. The cured elastomer compositions have a further propertyof a reduced tendancy to swell or increase in volume when immersed inflexographic inks, such as water based inks utilized in the flexographicprinting process. This low swell property is critical to the ultimateprinting quality which can be achieved with the printing plate sinceswelling causes the relief image to enlarge. A 2% dot for instance, dueto inordinate swelling of the plate in the printing ink may actuallyprint as a 15 or 20% dot on the printed page due to this undesirablephenomenon. The press life or wear life of the plate on press (thenumber of impressions until failure occurs) may also be greatly reducedby swelling which may result in a loss in physical strength of the curedelastomer compositon.

An additional feature of the radiation curable polyurethane elastomersof this invention is that they will cure or crosslink when exposed toactinic radiation without need of additional vinyl containing monomericadditives, such as monomers containing more than one vinyl group permolecule. This feature reduces some of the potential for contaminationof the environment of the earth (soil, water or air) with potentiallycytotoxic agents. Furthermore, the safety of the workers involved inproducing and using flexographic printing plates utilizing thistechnology is enhanced through elimination of human exposure to some ofthese same cytotoxic agents.

Further features of the radiation curable polyurethane elastomers ofthis invention are their relatively low melting temperatures and highmelt indices (low melt viscosities). The present invention particularlyconcerns flexographic printing plates produced from novel radiationcurable polyurethane elastomeric compositions, which printing plates aredeveloped utilizing a novel solventless absorption processing method.The absorption flexographic plate processing method eliminates the needfor liquid developers (water or solvent) of any kind. This novel platemaking method results in a substantial reduction of plate making steps,plate making process time, and the elimination of potentially toxicby-product waste streams in plate making. While it is foreseen that theradiation curable elastomeric polyurethane compositions according to thepresent invention may be used for other purposes, and in otherindustries, they are particularly well suited for application to theflexographic printing industry.

The storage stability of the radiation curable elastomer coated sheetmaterial prior to curing is also important. Specifically, resistance ofthe elastomer layer to cold flow during storage is desirable. If theelastomer layer undergoes too much cold flow, the resulting printingplate may lose its utility due to changes in plate thickness uniformitywhich could occur during such flow. The radiation curable polyurethaneelastomers of this invention are noted by a resistance and even a highresistance to cold flow prior to irradiation induced crosslinking orcuring.

Generally, a flexographic printing plate consists of a curableelastomeric polymer layer which is planar contiguous to a flexiblesupport layer or sheet which may be, for example, polymeric (film base)or metallic (metal base). Following cure of portions of the elastomericlayer (by imagewise exposure to actinic radiation), the uncured portionsare removed to reveal a relief structure (hills) which becomes theprinting surface of the plate in the flexographic printing process. Therelief structure (in the cured areas) is bound to the support layer byphysical and/or chemical action so as to remain in place during theprinting process through repeat impressions. The exposed surface of thecured elastomer layer becomes the ink receptive surface which bothreceives ink from the inking roll (anilox) and transfers it to thesubstrate to be printed during the printing process. The flexographicprinting process is a `direct` printing process because the printingplate and its temporarily bound ink layer are in direct contact with thesubstrate (e.g., paper or film) being printed. A variety of substancesmay be used as the support layer beneath the cured elastomeric layer ofa flexographic printing plate. Flexible substrates of syntheticpolymeric resins such as polyester, polycarbonate, or polyimide filmsmay be used or more rigid substrates (which are still flexible) such assteel or aluminum coil stocks may be selected.

DESCRIPTION OF THE INVENTION

The process of the present invention for producing a flexographicprinting plate comprises providing a relief imageable element comprisinga flexible substrate which can transmit ionizing radiation, saidsubstrate having on one surface thereof a radiation hardenablecomposition in a thickness of at least 0.3 mm, imagewise irradiatingsaid composition to harden the composition in irradiated areas,contacting said imagewise irradiated layer with an absorbent layer whichcan absorb unirradiated composition when it has been heated between 40°C. and 200° C., heating said composition layer to a temperature between40° C. and 200° C. while it is in contact with said absorbent layer,said temperature being sufficiently high so as to enable saidcomposition in unirradiated areas to be absorbed by said absorbent layer(usually by flowing into said absorbent layer), allowing at least 75% byweight of said composition (which is unirradiated) in unirradiated areasto be absorbed by said absorbent layer, and removing said absorbentlayer and said at least 75% by weight of composition from said flexiblesubstrate, the process further comprising the step of irradiating saidcomposition layer through the substrate with ionizing radiation toharden some but less than all of said composition layer and thereby forma hardened zone between said flexible substrate and unhardenedcomposition before said at least 5% of said composition is allowed to beabsorbed by said absorbent layer.

The step of irradiating the composition from the backside of the element(i.e., through the substrate) is preferably done before the imagewiseexposure to radiation to generate the floor or anchor zone beforehardening through the depth of the composition layer. The ionizingexposure may be performed after imaging with beneficial effects, butwithout as great a benefit as exposure prior to imaging.

A novel class of radiation curable polyurethane elastomers are providedwhich are derived from polymer forming reactions of: (A) at least onediisocyanate (e.g., a diisocyanate or mixture of diisocyanates); (B) afirst chain extension agent containing at least 2 free hydrogencontaining groups, preferably at least 2 hydroxyl groups and at leastone ethylenically unsaturated group per molecule; (C) an optionalingredient comprising a chain extension agent different from said firstchain extending agent and containing at least 2 free hydrogen containinggroups, preferably at least 2 hydroxyl groups and up to 11 carbon atoms;(D) a polyol having a molecular weight of at least 500 grams/mole whichcontains at least 2 free hydrogen containing groups such as at least 2hydroxyl groups per molecule (said polyol preferably but not essentiallyhaving a solubility parameter less about 9.0), which resultingpolyurethane elastomer has a number average molecular weight of at least10,000 grams/mole, a melt transition temperature of less than about 180°C. and a melt index at 180° C. of at least 0.5 grams/minute (accordingto ASTM No. D 1238-70) with an 11,000 gram load and a 0.38 inch bore.The elastomer of the present invention may also be generally describedas a radiation curable polyurethane elastomer having ethylenicallyunsaturated groups available for addition polymerization, said elastomercomprising the reaction product of

(A) 10-50% by weight of at least one diisocyanate,

(B) 0.5-20% by weight of a first chain extending agent having at leasttwo free hydrogen groups capable of polymerizing with isocyanate groups,and said first chain extending agent having at least one ethylenicallyunsaturated addition polymerizable group per molecule, and

(C) 10-70% by weight of an organic polyol having a molecular weight ofat least 500 and containing at least two free hydrogen containing groupscapable of polymerizing with isocyanate groups per molecule,

said polyurethane elastomer having a number average molecular weight ofat least 10,000 grams/mole, a melt transition temperature of less than180° C. and a melt index at 180° C. of at least 0.5 grams/minute. Theseelastomers may optionally contain 0.5 to 20% by weight (preferably 0.75to 12%) of a second chain extending agent different from said firstchain extending agent and having at least two free hydrogen groupscapable of polymerizing with isocyanate groups. The radiation curablepolyurethane elastomeric compositions of the present invention arecharacterized as belonging to a general class of polymers known assegmented copolymers or multiphase polymers, which general class orclasses have been well described in many references (e.g., MultiphasePolymers, Advances in Chemistry Series, Vol. 176, Stuart L. Cooper, andGerald M. Estes, editors, 1978, pp. 1-83). The elastomeric properties ofthis class of polymers results from phase segregation or phaseseparation between so called "hard" segment domains and "soft" segmentdomains. The "hard" segment domains, which possess a melt transitiontemperature or temperatures above the use temperature of the elastomer(when used, for example, as a printing plate, the use temperature wouldbe between about 15° C.-40° C.), act as reinforcing domains. The "soft"segment domains, which possess a glass transition temperature below theuse temperature of the elastomer, act as a toughening phase which isable to dissipate energy by a process known as viscous energydissipation. The two phases of the elastomer are thought to exist asseparate phases within the elastomer by virtue of thermodynamicincompatibility. The elastomers of the present invention are preferablyable to display a percent swell of less than 2%, more preferably lessthan 1% and most preferably less than 0.5% in water. This property isreadily measured by immersing the elastomer (e.g., 1 mm thick) indeionized water at 20° C. for twenty-four hours. If the thickness hasincreased less than 2%, there is less than 2% swell.

In the urethane elastomer having both hard segments formed by thereaction of components A), B), and/or C) and soft segments formed by thereaction of components A) and C), the soft segments comprise 20 to 70%by weight of the elastomer.

The novel radiation curable polyurethane elastomers which are producedfrom the above described polymer forming reactions are coated in layerform on a support sheet to provide an imageable photopolymer elastomerlayered product. Following imagewise irradiation induced curing of theelastomer layer, and removal of the uncured elastomer portion of theelastomer layer, a flexographic printing plate is provided which isparticularly useful in printing when combined with water basedflexographic printing inks. A particularly important feature of theradiation curable elastomeric polyurethanes of the present invention isthat the radiation curable feature derives from incorporation of theradiation curable functional groups into the polymer chains during thepolymer forming reactions. These radiation curable groups are retainedin the uncured state during the formation of the polymer chains,however, until activated by exposure to actinic radiation as will occurduring the flexographic printing plate making process. The radiationcurable functional groups are incorporated into the polymer chains bychain extending species (B).

The chain extension agents (B) useable according to the presentinvention include therein a reactive unsaturated moiety, preferablyparticularly sensitive to, and available for reaction upon curing viafree radical reactions to generate cross-linking. Preferred unsaturationmoities in chain extension agents according to the present invention arecarbon-carbon double bonds (olefinic or ethylenically unsaturatedbonds), and particularly preferred moieties are activated carbon-carbondouble bonds. Generally, "activated" carbon-carbon double bonds in aclass of chain extension agents utilizable according to the presentinvention include: double bonds activated through conjugation with acarbonyl group; those double bonds activated due to substitution byvarious other functional groups tending to stabilize free-radicalformation and hence activate the double bond toward free radicalreactions. In preferred chain extension agents according to the presentinvention, the "activated" double bond is oriented such that, when thechain extension agent is incorporated into the polymer backbone, i.e.,between urethane or carbamate units, the activated double bond is in aportion of the polymer molecule remote from, or pendant to, the polymerbackbone. That is, the activated double bond does not itself form partof the polymer backbone, but rather it is located on the polymer chainin a group pendant therefrom. A general chemical characteristic of chainextension agents according to the present invention is that they aredi-hydroxy substituted organic compounds, which contain at least oneactivated double bond therein. Preferably no more than two hydroxygroups (that is, preferably less than an average of 2.10 and morepreferably less than an average of 2.05 hydroxy equivalents percompound) are included in the compound, so that the chain extensionagents do not generate substantial networking and cross-linking, duringinitial reaction with a polyurethane prepolymer to form the extendedpolyurethane polymer composition (uncured). As previously noted, thesechain extending species may be di-free hydrogen containing compounds,which are inclusive of the di-hydroxy compounds. Useful free hydrogencontaining compounds would have such groups as --SH, --NH,, or --NHR(where R is an alkyl group or phenyl group) in place of one or more ofthe hydroxy groups. Hydroxy groups are especially preferred because ofthe resultant melting temperatures and melt indexes in the urethaneproduct. The other free hydrogen reactant groups can producepolyurethanes having too high of a melt transition temperature or toolow of a melt index. It is desirable to produce these polyurethanes withmelt transition temperatures less than 155° C. (preferably less than150° C.) and a melt index greater than 5 grams/10 minutes in thetemperature range of 100° C. to 180° C. when measured according to ASTMNo. D-1238-70 (preferably between 100° C. and 140° C.). Preferably, atleast one and, most preferably, both hydroxy groups of the chainextension agent (B) are primary, so that reaction with isocyanatemoieties to generate chain extension of growing polymer molecules willbe relatively rapid, so as to compete effectively in reaction with otherclasses of chain extension agents, described below, utilizable toprovide for other advantageous chemical and physical characteristics inthe elastomeric composition. Preferred chain extension agents are thosedescribed in U.S. Pat. No. 4,578,504 and U.S. Pat. No. 4,638,040 havingthe structural formula wherein --X and --Y are hydroxyl and hydrogenrespectively (1- or 2- glycerol acrylate or methacrylate) and thosedescribed in U.S. patent application Ser. No. 184,834 filed Apr. 22,1988, in the name of J. A. Martens, et al. such as N,N-bis(2-hydroxyethyl), N'-(methacryloxyethyl) urea:

    CH.sub.2 ═C(CH.sub.3)C(O)OCH.sub.2 CH.sub.2 NHC(O)N(CH.sub.2 CH.sub.2 OH).sub.2

In preferred compositions of the present invention, an activated doublebond-containing chain extension agent is incorporated into a polymer tobe cured such that, on the average, there is about 1 pendant, activated,unsaturation site provided every 2,000 to 10,000 molecular weight unitsin the polymer. For use in preferred applications such as flexographicprinting plates, the number average molecular weight of chains in thepolymeric elastomer (before curing) should be in the range of about20,000 to 50,000, corresponding to weight average molecular weights inthe range of about 35,000 to 95,000. In addition, the melt indices ofthe resulting molten radiation crosslinkable elastomeric compositionswhen measured according to ASTM No. D 1238-70 should be in the range of0.5 grams/minute to 10.0 grams/minute in the temperature range from 100°to 180° C.

To show how the percentage of double bond contributing reagent added tothe polymer affects the double bond equivalent weight of the polymer(the average molecular weight divided by the average number of doublebonds pendant from the polymer), consider a polymer having one percent(1%) by weight of a double bond providing reagent in the reactionmixture with a polymer whose molecular weight provides a double bondequivalent weight of 16,000 in the product. By doubling (to 2%) theweight of the double bond providing reagent, the double bond equivalentweight is halved to 8,000. Using 2-glycerol methacrylate (GMA) as anexample, in the same molecular weight polymer, the followingrelationship would exist for GMA versus equivalent weight (EW):

    ______________________________________                                               % GMA  EW                                                              ______________________________________                                               1%     16,000                                                                 2%     8,000                                                                  3%     5,333                                                                  4%     4,000                                                                  5%     3,200                                                           ______________________________________                                    

The polyol(D) of the reaction mixture, possessing a solubility parameterof less than about 9.0, is a hydrophobic polyol having as such aresistance to the imbibing of water and water/alcohol mixtures, and ahydroxyl equivalent weight of at least 250 gr./equivalent. Preferablepolyols which can be employed herein are those polyether polyols whichhave an average hydroxyl functionality of from about 2 to 3 and anaverage hydroxyl equvalent of from about 250 to about 5000, preferablyfrom about 500 to 3000, including mixtures of these polyols.Particularly suitable polyether polyols which can be employed includethose which are prepared by reacting an alkylene oxide or halogensubstituted alkylene oxide or mixtures thereof with an active hydrogencontaining initiator compound. Suitable such oxides include, for exampleethylene oxide, propylene oxide, 1,2-butylene oxide, 2,3-butylene oxide,epichlorohydrin, epibromohydrin, mixtures thereof and the like. Whenethylene oxide is employed, it should not constitute more than 10% byweight of the polyol. Preferred oxides are 1,2-butylene oxide andepichlorohydrin. Such polyols are well known in the art of polyurethanechemistry and are commercially available. The poly 1,2-(butylene oxide)polyols are commercial products of the Dow Chemical Company. Thepolyepichlorohydrin polyols are described in U.S. Pat. 4,431,845 and arecommercial products of the 3M Company.

The organic diisocyanate or diisocyanates (A) are selected from organicaromatic or aliphatic diisocyanates or mixtures thereof. Suitableorganic aromatic diisocyanates which can be employed include, forexample, any such isocyanate having 2 or more NCO groups per moleculesuch as for example 2,4-toluenediisocyanate, 2,6-toluenediisocyanate,p,p'-diphenylmethanediisocyanate, p-phenylene diisocyanate,naphthalenediisocyanate, polymethylene polyphenyl isocyanates,1,3,3,5-tetra methyl-1,6-bis(isocyanato)hexane (TMXDI), mixtures thereofand the like. Suitable organic aliphatic polyisocyanates include inaddition to the hydrogenated derivatives of the above mentioned organicaromatic polyisocyanates, 1,6 hexamethylene diisocyanate, 1,4-cyclohexyldiisocyanate, 1,4-bis-isocyanatomethyl-cyclohexane, isophoronediisocyanate, mixtures thereof and the like. Preferred isocyanates arethe cycloaliphatic diisocyanates which o include 4,4'-bis(isocyanatocyclohexyl) methane, 1,4-cyclohexyl diisocyanate, and isophoronediisocyanate.

Suitable chain extension agents (C) which can be employed includehydroxyl-containing aliphatic compounds which contain at least 2 but notmore than 3 hydroxyl groups per molecule. Suitable hydroxyl-containingchain extenders include, for example, ethylene glycol, propylene glycol,1,4 butane diol, diethylene glycol, dipropylene glycol, triethyleneglycol, tetraethylene glycol, glycerine, trimethylol propane, lowmolecular weight ethylene and/or propylene oxide derivatives ofglycerine, or trimethylol propane and the like.

The radiation curable elastomer composition may contain, in addition tothe above described components, other components or adjuvants such asphotoinitiators, colorants, fillers, catalysts, stabilizers andinhibitors, such as may be desired to impart other properties to thecurable elastomer layer such as storage stability, visible color, etc.Such adjuvants may be incorporated into the polymer backbone as byreaction or may simply exist as additives to the overall composition notincorporated into the polymer chain.

As is well known in the art, ethylenically unsaturated bonds (like thosewhich can be pendant from the elastomers used in the present invention)can be cured directly with ionizing radiation such as e-beams and otherhigh energy radiation). It is not essential that initiators be presentin the composition of the invention, but it is highly desirable.Preferably any of the known classes of photoinitiators, particularlyfree radical photoinitiators such as the quinones, acetophenones,benzoin ethers, aryl ketones, peroxides, biimidazoles, diaryliodoniums,triarylsulfoniums (and phosphoniums), diazoniums (especially thearomatic diazoniums), etc. may be used in the compositions of thepresent invention, generally in amounts of from 0.1 to 15% by weight,preferably from 0.3 to 10% by weight, more preferably from 0.5 to 8% byweight of the elastomer. The photoinitiators may be used as mixtures ofdifferent initiators and/or in combination with dyes that are well knownin the art to sensitize or spectrally sensitize the photoinitiators.

Another desirable aspect of preferred compositions useful in thepractice of the present invention is that lower molecular weightpolyacryloyl (including methacryloyl) materials are not needed in thecomposition. Such polyacryloyl materials (di-, tri-, tetra-, penta-,hexyl-acryloyl or methacryloyl) are undesirable for a number of reasons.The present invention has no problem operating completely free ofpolyacryloyl (which includes methacryloyl) compounds with molecularweights of 1000 or less. From many manufacturing standpoints it isespecially desirable to have crosslinkable compositions with less than2.0% by weight of such polyacryloyl compounds. The present inventionpreferably has less than 2.0% of such compounds, more preferably lessthan 1.0%, still more preferably less than 0.5%, and most preferably 0%of polyacryloyl materials with molecular weights less than or equal to1000.

The radiation curable polyurethane elastomeric compositions of thepresent invention are preferably produced utilizing melt polymerizationtechniques. Polymerization in the melt avoids the use of organicsolvents and solvent mixtures producing an elastomer product which canbe applied directly to the support sheet in molten form without concernfor drying ovens which would otherwise be necessary to remove suchorganic solvents from the elastomeric layer prior to making flexographicprinting plates. A preferred method of producing these compositionsinvolves use of a polymer processing melt extrusion device, such as atwin-screw counter rotating extrusion device, as the polymerizationreactor (such as disclosed in U.S. Pat. No. 4,843,134). Such anextrusion device provides for temperature control, thorough mixing ofreacting species, and control of the pressure profiles down the extruderduring processing which affects backmixing of and the resultingresidence time distribution of the reactants. Upon completion of theradiation curable elastomer forming reactions in the extruder, themolten, fully reacted, uncured thermoplastic elastomer composition canbe applied directly to the support sheet via passage through a filmforming extrusion die followed by contacting a moving web of the supportmaterials. The molten uncured elastomer also can be collected aspellets, or in some other solid form (slabs) for later processing intoflexographic printing plates using other forms of processing such asconventional single screw melt extrusion processing.

For the purpose of producing a flexographic printing plate havingexcellent durability (press life), and resistance to delamination of theelastomeric layer from the support layer or sheet, it is generallydesirable to enhance the adhesion or bonding strength of the curedelastomer layer to the support sheet utilizing priming agents oradhesion promoting treatments which are applied to the film or metallicsupport layer prior to application of the radiation curable elastomericlayer. Such treatments are generally done to the surface of the supportlayer or sheet prior to application of the molten curable elastomericlayer. Treatments such as corona discharge treatments, lasermodification as in U.S. Pat. 4,822,451, application of chemical primingagents, or mechanical roughening of the surface are effective inincreasing adhesion of the curable elastomeric layer to the support.

Following the completion of fabrication of the radiation curablepolyurethane elastomeric composition into a planar contiguous layer incontact with a support sheet (film or metallic base), flexographicprinting plates can be produced by imaging and development. This isaccomplished by curing the polyurethane elastomeric layer by exposure toactinic radiation, which exposure acts to harden or crosslink theelastomeric layer rendering it more or less unmeltable or insoluble inthe irradiated areas. The exposure to actinic radiation can be done inseveral ways. In a preferred method, th curing is accomplished in twoexposure steps. The first exposure, called the backside exposure, isdone with actinic radiation being directed toward and through thesupport layer into the elastomeric layer with the actinic radiationsource positioned ajacent to but seperated from the support layer sideof the two-layered plate being exposed. This exposure causes a gradationof curing or crosslinking of a portion of the curable elastomeric layer,the curing being most complete nearest the support layer. The timeduration of the backside exposure is determined experimentally and ischosen so as to cause a gradient of cure within the radiation-sensitiveelastomeric layer. The highest level of cure (crosslinking) of theelastomeric layer occurs nearest the support layer, with the cure levelsbeing reduced as the distance within the elastomeric layer, as measuredfrom the support layer, increases. A so-called gradient cure takes placeduring this exposure step, the gradient resulting from a fall off orreduction in effective actinic radiation intensity within differentlevels of the elastomeric layer. This reduction occurs by virtue of atleast partial absorption of actinic radiation by the elastomeric layer,as measured within the curable elastomeric layer. Following this briefexposure step (brief as compared to the imagewise exposure step whichfollows), an imagewise exposure is accomplished utilizing a photographicnegative mask which is placed in contact with the elastomer layer andthrough which actinic radiation is directed. This brief backsideexposure is preferably done with ionizing radiation and is not done withsuch radiation or intensity as would generally activate all free radicalphotoinitiators within the elastomeric layer. One can readily test thematerial to determine if there has been crosslinking or differentialcrosslinking by common testing procedures such as those shown in U.S.Pat. No. 4,576,850 (Gel Swell measurements).

A vacuum frame exposure device is preferred for such imaging (as opposedto the brief) exposure, which is accomplished following thoroughexhausting of air from between the elastomer layer and the photographicnegative. The exposure to actinic radiation in an imagewise manner isthen accomplished, with the exposure of sufficient duration as to renderthe curable elastomeric layer essentially unmeltable or insoluble underreasonable conditions of flexographic plate use.

In the most preferred method for forming the graded-cure radiationcurable elastomeric layer, the radiation curable elastomeric planarcontiguous layer is exposed to ionizing radiation from an electron beamsource with the accelerated electrons being directed toward and throughthe support layer into the elastomeric layer with an energy insufficientto penetrate the entire curable elastomeric layer.

Preferably less than 75% of the ionizing radiation would penetratethrough a thickness of 50% of the curable elastomeric layer. In thisway, the curing, or hardening of the elastomeric layer is only partiallyeffected with the curing being most complete at the interface of theelastomer layer with the support layer or sheet and being incomplete atthe outer surface (surface away from the support sheet) of the planarcontiguous elastomeric layer. By regulating or otherwise controlling thepenetrating power of the electron beam irradiation source, as bycontrolling the potential energy field through which the acceleratingelectrons are passed, the pathlength of the electrons through thesupport sheet and elastomeric layer is controlled. The backsidepre-imaging cure process step can provide a continuous, relatively thinlayer of cured elastomeric composition strongly bonded to the substrate.This thin cured layer can act as a footing or support surface for latergenerated image features. Particularly with respect to small details,e.g., 1 or 22 dots, the footing physically strengthens the adherence ofthe small features or details and reduces their tendency for wear orpremature removal from the plate. This increases the detailed printinglife of the flexographic plate.

In the preferred method, following electron beam exposure, an imagewiseexposure of the radiation curable elastomeric layer is accomplished.This exposure is most preferably accomplished by exposure of the curableelastomeric layer to actinic radiation through a photographic negativemask which is placed in contact with the elastomer layer and throughwhich actinic radiation is directed. A vacuum frame exposure device ispreferred for such exposure, which is accomplished following thoroughexhausting of air from between the elastomer layer and the photographicnegative. The exposure to actinic radiation in an imagewise manner isthen accomplished, with the exposure of sufficient duration as to renderthe curable elastomeric layer essentially unmeltable or insoluble.

Following imagewise exposure to actinic radiation, the development ofthe relief structure is accomplished by removal of the uncured portionsof the elastomer layer. In the preferred method of removal, an absorbantsheet material is utilized in the following manner. The photographicnegative mask is removed from the elastomer layer, and replaced with anabsorbant sheet material. The absorbant material is selected fromnon-woven web materials, paper stocks, fibrous woven web materials,open-celled foam materials, porour sheets, or other sheet materialswhich contain, more or less, a substantial fraction of their includedvolume as void volume. The uncured elastomer layer is heated byconduction, convection, or other heating methods to a temperaturesufficient to effect melting. By maintaining more or less intimatecontact of the absorbant sheet material with the molten elastomericlayer (molten in the uncured regions), a transfer of the uncuredelastomer from the planar contiguous layer to the absorbant sheetmaterial takes place. While yet in the heated condition, the absorbantsheet material is separated from the cured elastomer layer in contactwith the support layer to reveal the relief structure. After cooling toroom temperature, the resulting flexographic printing plate can bemounted on a printing plate cylinder and tested on a printing press asto printing capability.

Preferred absorbant sheet materials utilized to remove the uncuredportions of the elastomeric layer from the cured portions of said layerare selected from absorbant materials which possess internal strengthand tear resistance at temperatures up to, including, and slightlybeyond the melting temperature of the uncured radiation curablepolyurethane elastomeric composition, and which furthermore possess ahigh absorbancy for the molten elastomer composition as measured by thegrams of elastomer which can be absorbed per milliliter of absorbantmaterial. Preferred absorbant sheet materials, which may be referred toas development receptor sheets, are blown microfiber non-woven webmaterials produced from high temperature melting polymeric materialssuch as polypropylene, polyester, nylon or other high temperaturemelting thermoplastic polymers. The melting or softening temperature ofthe absorbant sheet material utilized should be higher than the meltingor softening temperature of the radiation curable polyurethane elastomerutilized in the planar contiguous layer of the flexographic printingplate being produced. Additional absorbant sheet materials which can beutilized according to the present invention include absorbant stocksproduced by various paper making processes. Absorbant materials such asopen-celled thermoset foams are also acceptable. Preferred absorbantsheet materials contain a void volume fraction of at least 50% of theincluded volume of the sheet (as measured in the uncompressedcondition). The most preferred absorbant sheet materials are spun-bondednylon non-woven webs such as CEREX™ non-woven webs produced by the JamesRiver Corporation. Inorganic filament webs, particularly those withporous filaments, may also be used.

In the use of the term absorption in defining the relative physicalproperty between the development receptor sheets and the melted uncuredelastomeric composition, no particular limitation on absorptivephenomena is intended. There need not be penetration of the meltedcomposition into the body of fibers, filaments or particles. Theabsorption into the bulk of the development receptor may be only bysurface wetting of the interior bulk. The driving force for the movementof the melted elastomeric composition into the absorptive area of thedevelopment receptor may be one or more of surface tension, electrical(e.g., van de Waals forces) forces, polarity attraction, or otherphysical forces known to assist in promoting philicity, adsorption orabsorption of materials.

In summary, the curable polyurethane elastomeric compositions accordingto the present invention comprise the reaction products of: 1) anorganic diisocyanate composition, which may include a mixture ofdiisocyanates; 2) a chain extension agent composition including aneffective amount of a difunctional hydroxyl reactant containing acarbon-carbon double bond; and 3) a polyol having a molecule weight ofat least 500 grams/mole and at least 2 hydroxyl groups which uponreaction of at least those three ingredients generates curablepolyurethane elastomeric compositions particularly useful in their curedforms as flexographic printing plate compositions. The invention alsodescribes flexographic printing plate materials comprising a curedpolyurethane elastomeric composition having adhered thereto a substrate,which flexographic printing plate materials are produced via asolventless absorption process method utilizing an absorbant sheetmaterial to remove the uncured elastomer portions of the adherantelastomer layer.

It is to be understood that the disclosures made herein are merelyexemplary of the invention, which may be embodied in various forms andsystems. Therefore, specific details disclosed herein are not to beinterpreted as limiting unless so specified. Rather, as the disclosureshould be considered support and a representative basis for teaching oneskilled in the art to variously practice the present invention inappropriate systems and manners.

The present invention includes within its scope: improved curablepolyurethane elastomeric resin compositions; certain cured elastomericcompositions; methods for producing improved flexographic printing platematerials from these curable elastomeric compositions; improvedflexographic printing plate materials produced from the improvedcompositions and an improved method of manufacture. The improvedpolyurethane elastomeric compositions according to the present inventionare generally suitable for curing upon exposure to irradiation, forexample ultraviolet (UV) or electron beam (EB) irradiation. Thecompositions are particularly well suited for use as flexographicprinting plates when used in conjunction with a support layer or sheetto which they are adhered.

Curable Polyurethane Elastomeric Compositions

The polyurethane elastomeric compositions according to the presentinvention include a chain extension agent incorporated into the backboneof a polyurethane polymer, the chain extension agent including therein,and preferably pendant to the polymer backbone, an unsaturation siteavailable for cross-linking the polymer upon irradiation cure.Typically, for preferred embodiments, the unsaturation site is anactivated carbon-carbon double bond. Preferred polyurethane elastomericcompositions useful according to the present invention are formed fromthe following constituents:

(a) an organic diisocyanate;

(b) a preferred chain extension agent according to the present inventionhaving incorporated therein an unsaturation site preferably displacedfrom the polymer backbone by a spacer group or groups so as to bependant to the polymer chain following incorporation;

(c) a dihydroxy functional chain extender, preferably containing 2-8aliphatic carbon atoms, as a reinforcing agent to promote toughness inthe polymer;

(d) a hydrophobic macroglycol or higher molecular weight polyol having amolecular weight in the range from about 500 to about 5000 grams/mole,which provides elasticity and energy dissipation capability in thepolymeric composition when incorporated between the rather hard urethanesegments.

It should be readily understood that the above four constituents may bevaried in structure and relative amounts in the curable compositions, ina manner permitting production of elastomeric compositions having a widevariety of chemical and physical performance properties. Desired uncuredand cured properties can be readily obtained, and predictably andconsistently generated. For example, by varying the relative amounts ofdiisocyanate (a), and short chain extending diols (b) and (c) as aproportion by weight of the total mass of the curable composition, thedurometer or Shore hardness of the elastomer can be varied andcontrolled. Furthermore, by varying the proportion by weight of thechain extending species (b) to the total mass of the elastomercomposition, the cross-link density (average molecular weight betweencrosslink sites) can be controllably and predictably varied in the curedcomposition.

Organic diisocyanates useable in forming elastomeric polyurethanecompositions according to the present invention may be of a variety oftypes. Generally, aromatic or cycloliphatic diisocyanates having anaverage molecular weight of about 160 to about 450 are preferred. Theseinclude, for example:

4,4'-di(isocyanatophenyl) methane

4,4'-di(isocyanatocyclohexyl) methane

2,4-toluene diisocyanate

2,6-toluene diisocyanate

p-phenylene diisocyanate

1,4-di(isocyanato) cyclohexane

isophorone diisocyanate

The remaining above described chemical reactant constituents utilized informing the elastomeric polyurethane compositions generally comprisediols, each having preferred characteristics to impart certain desiredchemical and physical properties to the resulting polyurethaneelastomer. A first of these is a chain extending diol component having arelatively low hydroxyl equivalent eight, mentioned above as component(c), which reacts with isocyanate moities to form relativelyshort-chain, hard, tough, segments in the backbone of the polyurethanepolymer. Preferred diol components (c) are: 1,2-ethylene glycol;propylene glycol; 1,4-butane diol; diethylene glycol; dipropyleneglycol; and triethylene glycol.

The relatively high molecular weight macroglycol (polyol) component (d)utilized in preparing the polyurethane elastomeric compositionsaccording to the present invention are extended chain diols, which,following reaction act as soft, low Tg, energy dissipating, hydrophobicsegments in the resulting polyurethane elastomers. Suitable macroglycolswhich can be utilized are those polyether polyols having an averagehydroxyl functionality of 2 to 3 and an average hydroxyl equivalentweight of from about 250 to about 5000, preferably from about 1000 toabout 3500. Suitable polyether polyols which can be employed includethose which are prepared by reacting an alkylene oxide, or halogensubstituted alkylene oxide or mixtures thereof with an active hydrogencontaining compound. Prefered oxides are 1,2-butylene oxide andepichlorohydrin either alone or in combination.

Improvements according to the present invention can also result fromincorporation of yet a third class of diol compound into the polymerforming reaction mixture. This optional third class of diol compound maybe used to introduce further characteristics to the resultingpolyurethane elastomeric composition. In particular, the third class ofcomponent comprises a relatively low molecular weight chain extensionagent which is difunctional in hydroxyl groups and also includes anunsaturated moiety, generally pendant to the resulting polymer chain. Ina surprising aspect, by judicious selection of this species withparticular regard for its structural features, this unsaturated diolprovides a dual function of being both a curable moiety which providesfor radiation curability but also, and just as important, a reduction ofthe melting temperature of the elastomeric composition to temperaturesas low as about 85° C. These features further enable the production of aflexographic printing plate produced via irradiation induced curing ofthe elastomeric composition, followed by removal of the uncrosslinkedelastomer portions by absorption by an absorbant material attemperatures above the melting point of the uncured elastomericcomposition, which temperatures are well below the heat distortiontemperatures of the support layer beneath the cured elastomeric layer inthe printing plate. Preferably the unsaturated diol comprises anunsaturation site with an activated carbon-carbon double bond. Thisactivated double bond is thus readily available and particularlyreactive for cross-linking reaction upon exposure to activatingirradiation.

The term "activated", or variants thereof when used in association withan unsaturated moiety of an agent according to the present invention,refers to a basic type of unsaturation site which is activated in thesense that free radicals formed thereat will be stabilized by othergroups in the molecule, hence facilitating the free radical reaction.Such activated double bonds include, for example, the carbon-carbondouble bonds of alpha-beta unsaturated carbonyl compounds, for exampleacrylates, methacrylates, and acrylamides.

Preferred activated unsaturation moiety-including chain extending diolsinclude: 1-, or 2-glycerol acrylate or methacrylates; trimethylolethanemonoacrylate or methacrylate; and N,N-bis(2-hydroxyethyl)-N'-(methacryloxyethyl) urea.

In curable elastomeric compositions according to the present invention,generally at least 1 activated unsaturation moiety should be introducedfor about every 500-15,000 or preferably every 2,000-10,000 molecularweight units of elastomer polyurethane. This can be readily controlledand repeated and is a function of the reactivity and/or amount(s) ofreactant(s) used in forming the polyurethane. Substantially higherquantities of unsaturation moiety may lead to a relatively highlycrosslinked, and therefore brittle, cured composition. Such acomposition may have utility in many applications, but not typicallythose involving a resilient flexographic printing plate. Substantiallyless incorporation of unsaturated moieties will generally result ininsufficient cross-linking or cure of the elastomer as required toresist flow above the elastomer melt transition temperature which flowwill result in permanent undesired distortion of the printing platesurface rendering the printed matter produced from it as unacceptable.

Formation of the Curable Elastomeric Compositions

The curable elastomeric compositions can be produced by any of a numberwell known solvent based or melt polymerization techniques. Meltpolymerization techniques such as one shot, pre-polymer with handcasting, reaction injection molding, or reactive extrusion techniques,are preferred since these methods can eliminate solvents and solventdrying.

Particularly preferred techniques are those involving reactive extrusionutilizing multi-screw extrusion equipment such as counter rotatingtwin-screw extrusion equipment.

The percent by weight change of the cured elastomer composition whenimmersed in various liquids is determined by the following procedure:

Test strips of cured elastomer of dimensions 12.7 mm×25.4 mm×0.4 mm areweighed to the nearest 0.0001 gram (S1). The strips are then placed insufficient liquid as to be fully immersed and equilibrated for 24 hours.After careful removal using a forceps, excess liquid is removed from thetest strip using an absorbant towel, and the strip is reweighed to thenearest 0.0001 gram (S2). The percent of swelling (% Swell) iscalculated by the following equation: ##EQU1##

The percentage of gell (% Gel) after curing the elastomer compositionsis determined by the following procedure:

Test strips 12.7 mm×102 mm×0.4 mm are weighed to the nearest 0.0001 gram(G1). The strips are immersed in 50 grams of tetrahydrofuran and allowedto soak for 24 hours. The swollen strips are carefully removed from thesolvent using a forceps, excess solvent is removed from them with anabsorbent material and the strips are placed in a pre-weighed aluminumpan. The strips are allowed to air dry for 8 hours followed by dryingfor 8 hours in a vacuum oven maintained at 80° C. and 29 inches ofmercury vacuum. The strips/pan are re-weighed with the gel weight (G2)being determined by difference. The % Gel is calculated as follows:##EQU2##

The storage modulus, E', loss modulus, E", and loss tangent σ, of theelastomeric compositions are determined utilizing commercially availablemeasuring equipment, in this case a Rheovibron Analyzer (product of theToyo-Baldwin Co.--Tokyo, Japan) using the following procedure:

Test strips of approximately 3.8 mm×70 mm×0.4 mm are placed between thejaw clamps of the Rheovibron apparatus. The sample is tensioned slightlyand then cooled to -50° C., at which point a sinusoidal tension isapplied from the driver side of the apparatus at a frequency of 11 Herzwhile the sample temperature is elevated at the rate of +3° C. perminute. The resulting tension applied through the sample to the othersupport arm and its phase shift in degrees from the driving tension aremeasured and used to calculate E', E", and Tan σ (tangent of the angleσ).

The melt index of the molten uncured elastomer compositions isdetermined according to ASTM No. D 1238-70 as follows. Five (5.0) gramsof elastomer pellets are loaded into the heated chamber of aTinius-Olsen Extrusion Plastometer, which chamber is equilabrated at atemperature of 153°+/-0.5° C. A load of 1100 grams mass is applied tothe melting elastomer in the chamber, which load acts to force themolten elastomer composition from the small orifice at the bottom of thePlastometer chamber. After 5 minutes, during which period the polymerflow tends toward equilibrium, a sample of extruded elastomer iscollected over a 20 second interval and weighed as M1. The melt index isthen calculated using the following equation:

    Melt Index=M1×30 [having the units of grams/10 minutes]

EXAMPLE 1A Curable Elastomer Composition from 2-Glycerol MethacrylateUnsaturated Diol and Poly 1,2-(butylene oxide) Diol

A radiation curable polyurethane elastomeric composition was prepared ina twin-screw extrusion reactor as follows:

I. A polyol mixture was prepared of the following components which werethoroughly mixed in a feed tank until homogenious:

A. 286.1 parts (0.2861 moles) of a 1000 molecular weight poly1,2-(butylene oxide) diol (Dow Chemical Co.);

B. 32.8 parts (0.3644 mole) of 1,4-butane diol (GAF Chemical Co.)

C. 10.7 parts (0.0669 mole) of 2-glycerol methacrylate (3M Co.);

D. 10.6 parts of diethoxy acetophenone (Irgacure-651,Ciba-Geigy Co.)

E. 0.1 part methylene blue

F. 0.06 part ferric chloride

G 0.26 part dibutyl tin dilaurate

II. A precision flow metering system was utilized to meter the abovepolyol stream into the inlet port of a 64 mm twin-screw counter rotatingextruder (Leistritz Co.) at a ratio of 62.47 parts by weight of polyolstream to 37.53 parts by weight 4,4'-bis(isocyanato cyclohexyl) methane(Desmodur W™, Mobay Chemical Co.). At this mass ratio, there was aslight equivalency excess of isocyanate moities in the feed streamrelative to hydroxyl moieties. The reaction temperature of thepolymerizing mass was maintained in the range of 150° to 170° C. aspolymerization occurred in the extruder. Upon exiting from the extruder,the fully reacted curable elastomer composition was segregated intopellets having diameters of approximately 0.3 cm which were collectedfor further processing. The completeness of the polymerization reactionwas determined by monitoring the infrared spectrum of a cast film of thecurable elastomer product and determining the absorbance ratio of the--NCO absorption band (2250 cm-1) to the --CH2-- absorption band(2950cm-1). A ratio of less than 0.2 indicated a complete reaction withonly a slight excess of --NCO groups remaining. The melt index of thecurable elastomer composition was monitered at a temperature of 153° C.using a load of 1100 grams on the heated chamber of the ExtrusionPlastometer and found to be in the range between 10 and 20 grams per 10minute interval.

The fully reacted curable polyurethane elastomeric Composition 1Aconsisted of the following mole ratios of constituents:

    ______________________________________                                        Component               Moles                                                 ______________________________________                                        4,4'-bis(isocyanato cyclohexyl)methane                                                                2.730                                                 1,4 butane diol         1.274                                                 2-glycerol methacrylate 0.234                                                 poly 1,2 (butylene oxide)diol                                                                         1.000                                                 ______________________________________                                    

EXAMPLE 1B Curable and Cured Elastomeric Films, and FlexographicPrinting Plates from Composition 1A

The radiation curable polyurethane elastomeric Composition 1A describedin Example 1A was reextruded into a curable flexographic printing plateconstruction utilizing a 125 mm single screw extrusion device asfollows. Curable elastomer pellets of Composition 1A were charged intothe feed hopper of the extruder. The temperatures of the heated zones ofthe extruder were maintained between 130° and 160° C. during theexperiment. A film extrusion die was utilized at the exit of theextruder to allow casting of the extrudate onto a polyethyleneterephthalate film base of 0.18 mm thickness to form the planarcontiguous curable elastomeric layer of the flexographic printing plate.Prior to beginning the extrusion step, the major portion of the filmbase had been coated with a priming composition comprising atris-aziridine compound (as disclosed in EPO Publication 0 206 669) toenhance adhesion of the extruded layer. The extrudate was introducedinto a controlled orifice gap consisting of two rotating chill rollsmaintained at 20°-25° C. An unprimed top film of polyethyleneterephthalate of 0.08 mm thickness was introduced into this gap also toserve as a protective film over the curable elastomeric layer prior toformation of the flexographic printing plate by irradiation inducedcuring. A portion of the extrudate was applied to unprimed 0.18 mm filmbase for the purpose of measuring % Gel, and % Swell after cure andphysical properties (tensile strength and modulus at room temperature,and dynamic thermomechanical properties).

A continuous roll of multi-layered product of thickness 0.66 mm was thusproduced having a curable elastomeric layer of Composition 1A ofthickness 0.4 mm more or less in combination with a support sheet ofpolyethylene terephthalate film of 0.18 mm thickness and a removable topfilm of 0.08 mm thickness.

A portion of the extruded curable elastomeric composition which had beenapplied to unprimed film base was cured by exposure for 10 minutesduration at an intensity of 11,200 millijoules/cm² to actinic radiationutilizing a vacuum frame ultraviolet light exposure unit (Kelleigh Model#210 flexographic plate exposure unit). Physical properties before andafter cure were evaluated by tensile strength testing according to ASTMD 1708 at a crosshead speed of 30 cm/minute. Results are summarized inTable I. Dynamic thermomechanical testing to determine storage modulusE', loss modulus E", and Tan σ was done as previously described. Theresults are graphically displayed in FIG. 1. The % Gel was determined tomeasure the degree of insolubility produced by radiation curing and issummarized in Table II. The % Swell of the cured elastomeric compositionafter immersion in water and water/alcohol mixtures for 24 hours wasdetermined as a measure of the resistance of that composition againstswelling in various water based ink formulations, and is summarized inTable III.

                  TABLE I                                                         ______________________________________                                                    Physical Properties of                                                        Uncured & Cured Films                                                      Tensile  Elastic                                                              Strength Modulus   Modulus                                           Example  kg/cm2   kg/cm2    kg/cm2 % Elongation                               ______________________________________                                        1B-uncured                                                                             280.4    208.5     43.7   510.0                                      1B-cured 267.7    105.7     59.9   343.0                                      2B-uncured                                                                             201.5    199.4     42.0   500.0                                      2B-cured 208.5    210.7     71.9   276.4                                      3B-uncured                                                                    3B-cured                                                                      ______________________________________                                    

The data demonstrate the elastic properties of the uncured and curedelastomeric compositions.

                  TABLE II                                                        ______________________________________                                                  % Gel (24 hours extraction in THF)                                  Example   after UV Curing Exposure                                            ______________________________________                                        1B-uncured                                                                              0                                                                   1B-cured  79.2                                                                2B-uncured                                                                              0                                                                   2B-cured  92.1                                                                3B-uncured                                                                              0                                                                   3B-cured                                                                      ______________________________________                                    

The data demonstrate the conversion during radiation exposure of theuncured elastomeric compositions to relatively insoluble curedcompositions having % Gel contents in excess of 70%.

                  TABLE III                                                       ______________________________________                                                   % Swell (24 hours immersion in                                                98/2 H.sub.2 O/ 95/5 H.sub.2 O/ 87.5% H.sub.2 O/)                  Example                                                                              100% H.sub.2 O                                                                          glycerine n-propanol                                                                            isopropanol*                               ______________________________________                                        1B-cured                                                                             0.78      0.83      1.78    1.8                                        2B-cured                                                                      3B-cured                                                                      ______________________________________                                         *pH adjusted to 9.2 with ammonia (aq).                                   

The data demonstrate the excellent resistance to swelling of the curedelastomeric compositions of the present invention in various water andwater/alcohol mixtures as representative of water based flexographic inkformulations.

One-meter long sections of the multi-layered product described in theprevious two paragraphs were irradiated with ionizing radiation from anElectrocurtain™ electron beam irradiation device (product of EnergySciences, Inc.) as follows. The accelerating potential of the electronseminating from the unit was preset to 240 kiloelectron volts. Theproduct section was exposed to the electron beam energy in anorientation so that the beam energy was directed toward the product fromthe 0.4 mm polyester film support side. In this manner, the portion ofthe curable elastomer layer in contact with the primed polyester filmbase received the greatest irradiation energy. The energy dose wascontrolled so that the product received an absorbed dose of 5 megaradsas measured at the point at which the beam entered the product surface.This exposure step was accomplished over the entire product area so asto partially cure, or render unmeltable, and insoluble, a portion of thecurable elastomeric layer, particularly that portion in direct contactwith the polyester support base.

An imagewise exposure of the planar contiguous elastomeric layer wasnext accomplished as follows. The polyester top film (0.08 mm) wasremoved from the elastomeric layer of the curable elastomeric layer. Athin coating of a water dispersed urethane resin which contained smallbeads of silicon dioxide of approximately 20 microns in diameter wasapplied to the exposed surface of the curable elastomeric layer andallowed to air dry for a few minutes. A silver halide photographicexposure negative (of the type in common use in the graphic artsindustry) which contained picture information in the form of the magentaseparation obtained from a 35 mm photographic slide of a clown (whichseparation was produced utilizing a film scanner (Hell Corporation) at52.4 line screen per inch definition) was placed in contact with theurethane resin/silicon dioxide coated side of the curable elastomericlayer. This multilayered "sandwich" was placed in the vacuum exposureframe contained in a Kelleigh flexographic plate processor (Model #210).The top film attached to the exposure frame was drawn over the"sandwich", vacuum was applied, and the air was exhausted between theexposure negative and the curable elastomer surface. Ultraviolet lightexposure of the plate through the photographic negative was thenperformed for a 6 minute period, after which the evacuation wasterminated, and the exposure negative removed. A visible image was notedin the UV exposed areas of elastomeric layer (photobleaching hadoccurred which rendered the exposed areas transparent and a light yellowcolor) while the unexposed areas remained light blue in color. Removalof the unexposed and uncured areas of the elastomeric layer (to completethe manufacture of a flexographic printing plate) was next accomplishedas follows. Sections of non-woven spun-bonded nylon porous web (CEREX™spunbonded nylon, a product of James River Corp.) of basis weight 66grams/square meter were cut in size to match the area of the plate to beprocesssed. A layer of the non-woven web was placed in contact with theelastomer layer of the exposed plate. The laminate was placed on aheated platten equilibrated to 135° C. with the polyester film surfaceof the plate in contact with the platten. Directly adjacent to theplatten were two heated, rubber covered, nip rolls which were moving incounter-rotation at a linear speed of 30 cm/minute and which were gappedso as to lightly compress the laminate of non-woven/plate as it wasintroduced into the nip roll gap. After a few seconds of warm-up time onthe platten, the laminate was gently pushed into the nip roll gap. Asthe "sandwich" eminated from the heated nip, the CEREX™ non-woven webwas gently lifted from the heated elastomer surface with steady tension.It was noted that the non-cured areas of the elastomer layer of theplate had been removed via absorption of the thermoplastic uncuredportions of the elastomer into the non-woven web. A half-tone image ofthe clown at 52.4 lines/cm was evident in the cured elastomeric layer ofthe plate. Two additional trips of the cured product through the heatednip of the laminator with fresh CEREX™ non-woven web sections wererequired to complete the removal of the unexposed areas of the plate. Ina similar manner, the other photographic negative color seperations(black, cyan, yellow) of the clown slide were processed intoflexographic printing plates for use in color printing.

Printing was accomplished utilizing a 5 station Webtron™ Model 525flexographic printing press and water based flexographic printing inks(Louis Werneke Co.) with a tag and label printing base being utilized.Printing was done under standard conditions utilized in traditionalflexographic printing practice. An excellent rendition of the clownpicture was reproduced in this way.

To further demonstrate the utility and value of the electron beamexposure, and resulting curing step, the following experiment wasconducted. Sections of the extruded curable elastomeric compositionwhich had been applied to the primed polyester film base were exposed toionizing radiation from an Electrocurtain™ electron beam exposure deviceas previously described. The beam to product orientation was againarranged so as to allow the ionizing beam of radiation to enter theproduct through the 0.4 mm film support side. In this case however theaccelerating voltage of the electrons was varied over the range from 240KeV to 280 KeV in steps of 10 KeV, while the dose was maintained at 5megarads. Following exposure to ionizing radiation, the uncured portionsof the elastomeric layer were removed by adsorption into CEREX™non-woven nylon web as described above in the production of theflexographic printing plates containing the clown seperations. Afterremoval of the uncured elastomer was complete, the remaining caliper ofcured elastomer and its adherent polyester film base was determined bymeasurement utilizing a digital micrometer. A total of four measurementsof each sample was made to allow determination of error in the methods.The thickness measurement is referred to as the floor thickness of theplate and will be that thickness down to which removal of uncuredelastomer will be successful following imagewise exposure of theflexographic plate. The total caliper of the printing plate iscontrolled by the extrusion process and is set by imagewise exposure.The relief depth of the printing areas of the plate is determined bycalculating the difference between the total plate caliper and the floorthickness of the plate. For example, for a total plate caliper of 22mils, and a pre-set floor of 16 mils (as obtained from exposure toelectron beam irradiation utilzing a beam having 240 KeV of penetratingpower), a relief of 6 mils would be obtained in the final flexographicprinting plate. The use of ionizing electron beam radiation exposurethus allows the manufacturer to control the floor thickness of aflexographic plate in a systematic way. Further unexpected benefits ofthe electron beam exposure step will become apparent in further examples

EXAMPLE 2A Curable Elastomeric Composition fromN,N-Bis(2-hydroxyethyl),N'-(methacryloxy ethyl) urea unsaturated dioland poly 1,2-(butylene oxide) diol

Preparation of Unsaturated diol:

A 10 gallon glass lined Pfaudler chemical reaction vessel was utilizedfor the reaction. The following materials were used.

    ______________________________________                                        Component             Kg.      Kg-Moles                                       ______________________________________                                        A. Diethanol amine (Dow Chemical Co.)                                                                7.22    0.0687                                         B. poly 1,2-(butylene oxide)diol (Dow)                                                               9.09    0.0091                                         C. 2-isocyanato ethyl methacrylate                                                                  10.96    0.07                                           (distilled under vacuum - Dow                                                 Chemical Co.)                                                                 D. Phenothiazine      8.2 grams                                                                              --                                             ______________________________________                                    

Components A and B were charged to the vessel under vacuum and cooled toa temperature of 20° C. with agitation. Component D was added while anitrogen gas blanket was established in the vessel after which componentC was slowly metered into the reactor while cooling the reactor jacketwith cold water. The addition rate was controlled so as to maintain thereaction temperature below 40° C. The addition of component C wascomplete after 1 hour of addition time. After allowing 15 minutes forthe reaction to be completed, a small sample of the reaction mixture waswithdrawn and examined by infrared spectroscopy for the presence ofresidual isocyanate moieties. A very slight absorption band at 2250 cm-1was noted, indicating the desired slight excess of --NCO in the reactionmixture. The contents of the reactor were drained into lined pails andheld for further use as described below. The resultingN,N-Bis(2-hydroxyethyl)-N'-(methacryloxy ethyl)urea unsaturated diolproduct was further characterized by 1H and 13C NMR to establish itsstructure and purity.

A radiation curable polyurethane elastomeric composition was preparedfrom the above unsaturated diol as follows.

I. A polyol mixture was prepared of the following components which werethoroughly mixed in a feed tank until homogenious:

A. 260 parts (0.26 moles) of a 1000 molecular weight poly 1,2-(butyleneoxide) diol (Dow Chemical Co.);

B. 29.7 parts (0.33 mole) of 1,4-butane diol (GAF Chemical Co.)

C. 22.97 parts of reaction product above containing 0.0606 mole ofN,N-Bis (2-hydroxyethyl)-N'-(methacryloxy ethyl)urea and 0.0007 mole ofpoly 1,2(butylene oxide) polyol

D. 5.0 parts of diethoxy acetophenone (Irgacure-651, Ciba-Geigy Co.)

E. 0.1 part methylene blue

F. 0.06 part ferric chloride

G. 0.25 part dibutyl tin dilaurate

II. A precision flow metering system was utilized to meter the abovepolyol stream into the inlet port of a 64 mm twin-screw counter rotatingextruder (Leistritz Co.) at a ratio of 63.17 parts by weight of polyolstream to 36.83 parts by weight 4,4'-bis (isocyanato cyclohexyl) methane(Desmodur™ W- Mobay Chemical Co.). At this mass ratio, there was aslight equivalency excess of isocyanate moities in the feed streamrelative to hydroxyl moities. The reaction temperature of thepolymerizing mass was maintained in the range of 150° to 170° C. aspolymerization occurred in the extruder. Upon exiting from the extruder,the fully reacted curable elastomer composition was segregated intopellets of diameter of approximately 0.3 cm which were collected forfurther processing. The completeness of the polymerization reaction wasdetermined by monitoring the infrared spectrum of a cast film of thecurable elastomer product and determining the absorbance ratio of the--NCO absorption band (2250 cm-1) to the --CH2--absorption band(2950cm-1), which ratio proved to be less than 0.2 indicating a completereaction with only a slight excess of --NCO groups remaining. The meltindex of the curable elastomer composition was monitered at atemperature of 153° C. using a load of 1100 grams on the heated chamberof the Extrusion Plastometer and found to be in the range between 5 and10 grams per 10 minute interval.

The fully reacted curable polyurethane elastomeric Composition 2Aconsisted of the following mole ratios of constituents:

    ______________________________________                                        Component               Moles                                                 ______________________________________                                        4,4'-bis(isocyanato cyclohexyl)methane                                                                2.730                                                 1,4-butane diol         1.266                                                 N,N-Bis (2-hydroxyethyl),                                                                             0.233                                                 N'-(methacryloxy ethyl)urea                                                   poly 1,2-(butylene oxide)diol                                                                         1.000                                                 ______________________________________                                    

By calculation, Composition 2A possesses one unsaturated crosslinkingsite per every 5,167 molecular weight units.

EXAMPLE 2B Curable and Cured Elastomeric Films, and FlexographicPrinting Plates from Experiment 2A, Composition 2A

The radiation curable polyurethane elastomeric Composition 2A describedin Example 2A was reextruded into a curable flexographic printing plateconstruction utilizing a 125 mm single screw extrusion device asfollows. Curable elastomer pellets of Composition 2A were charged intothe feed hopper of the extruder. The temperatures of the heated zones ofthe extruder were maintained between 130° and 160° C. during theexperiment. A film extrusion die was utilized at the exit of theextruder to allow casting of the extrudate onto a polyethyleneterephthalate film base of 0.18 mm thickness for the purpose of formingthe planar contiguous curable elastomeric layer of the flexographicprinting plate. Prior to beginning the extrusion step, the major portionof the film base had been coated with a priming composition to enhanceadhesion of the extruded layer. The extrudate was introduced into acontrolled orifice gap consisting of two rotating chill rolls maintainedat 20°-25° C. An unprimed top film of polyethylene terephthalate of 0.08mm thickness was introduced into this gap also to serve as a protectivefilm over the curable elastomeric layer prior to formation of theflexographic printing plate by irradiation induced curing. A portion ofthe extrudate was applied to unprimed 0.18 mm film base for the purposeof measuring % Gel, and % Swell after cure and physical properties(tensile strength and modulus at room temperature, and dynamicthermomechanical properties).

A continuous roll of multi-layered product of thickness 0.66 mm was thusproduced having a curable elastomeric layer of Composition 1A ofthickness 0.4 mm more or less in combination with a support sheet ofpolyethylene terephthalate film of 0.18 mm thickness and a removable topfilm of 0.08 mm thickness.

A portion of the extruded curable elastomeric composition which had beenapplied to unprimed film base was cured by exposure for 10 minutesduration to actinic radiation utilizing a vacuum frame ultraviolet lighto exposure unit (Kelleigh Model #210 flexographic plate exposure unit).Physical properties before and after cure were evaluated by tensilestrength testing according to ASTM D 1708 at a crosshead speed of 30cm/minute. Results are summarized in Table I. Dynamic thermomechanicaltesting to determine storage modulus E', loss modulus E", and Tan σ weredetermined by Rheovibron™ analysis and are summarized as follows: PerCent (%) Gel was determined to measure the degree of insolubilityproduced by radiation curing and is summarized in Table II. The % Swellof the cured elastomeric composition after immersion in water andwater/alcohol mixtures for 24 hours was determined as a measure of theresistance of that composition against swelling in various water basedink formulations, and is summarized in Table III.

One-meter long sections of the multi-layered product described in theprevious two paragraphs were irradiated with ionizing radiation from anElectrocurtain™ electron beam irradiation device (product of EnergySciences, Inc.) as follows. The accelerating potential of the electronseminating from the unit was preset to 240 kiloelectron volts. Theproduct section was exposed to the electron beam energy in anorientation so that the beam energy was directed toward the product fromthe 0.4 mm polyester film support side. In this manner, the portion ofthe curable elastomer layer in contact with the primed polyester filmbase received the greatest irradiation energy. The energy dose wascontrolled so that the product received an absorbed dose of 5 megaradsas measured at the point at which the beam entered the product surface.This exposure step was accomplished over the entire product area so asto partially cure, or render unmeltable, and insoluble, that portion ofthe curable elastomeric layer in direct contact with the polyestersupport base.

An imagewise exposure of the planar contiguous elastomeric layer wasnext accomplished as follows. The polyester top film (0.08 mm) wasremoved from the elastomeric layer of the curable elastomeric layer. Athin coating of a water dispersed urethane resin which contained smallbeads of silicon dioxide of approximately 20 microns in diameter wasapplied to the exposed surface of the curable elastomeric layer andallowed to air dry for a few minutes. A silver halide photographicexposure negative (of the type in common use in the graphic artsindustry) which contained graphic arts test patterns with half-tone dotsfrom 65 to 150 line screen at % dot densities from 2% to 95% was placedin contact with the urethane resin/silicon dioxide coated side of thecurable elastomeric layer. This multilayered "sandwich" was placed inthe vacuum exposure frame contained in a Kelleigh flexographic plateprocessor (Model #210). The top film attached to the exposure frame wasdrawn over the "sandwich", vacuum was applied, and the air was exhaustedbetween the exposure negative and the curable elastomer surface.Ultraviolet light exposure of the plate through the photographicnegative was then accomplished for a 6 minute period, after which theevacuation was terminated, and the exposure negative removed. A visibleimage was noted in the UV exposed areas of elastomeric layer(photobleaching had occurred which rendered the exposed areastransparent and a light yellow color) while the unexposed areas remainedlight blue in color. Removal of the unexposed and uncured areas of theelastomeric layer (to complete the manufacture of a flexographicprinting plate) was next accomplished as follows. Sections of non-wovenspun-bonded nylon porous web (CEREX™ spunbonded nylon, a product ofJames River Corp.) of basis weight 66 grams/square meter were cut insize to match the area of the plate to be processsed. A layer of thenon-woven web was placed in contact with the elastomer layer of theexposed plate. The laminate was placed on a heated platten equilibratedto 135° C. with the polyester film surface of the plate in contact withthe platten Directly adjacent to the platten were two heated, rubbercovered, nip rolls which were moving in counter-rotation at a linearspeed of 30 cm/minute and which were gapped so as to lightly compressthe laminate of non-woven/plate as it was introduced into the nip rollgap. After a few seconds of warm-up time on the platten, the laminatewas gently pushed into the nip roll gap. As the "sandwich" eminated fromthe heated nip, the CEREX™ non-woven web was gently lifted from theheated elastomer surface with steady tension. It was noted that thenon-cured areas of the elastomer layer of the plate had been removed viaabsorption of the uncured thermoplastic portions of the elastomer intothe non-woven web. A flexographic printing plate was produced having ahalf-tone relief image which matched the pattern of the exposurenegative. Two additional trips of the cured product through the heatednip of the laminator with fresh CEREX™ non-woven web sections wererequired to complete the removal of the unexposed areas of the plate.

Printing was accomplished using a 0.4 meter wide GALLUS Comcoflexogrpahic printing press, water based flexographic printing inks(Louis Werneke Co.) and a pressure sensitive adhesive backed tag andlabel printing substrate. Excellent tonal reproduction of the originaltest negative was noted in the printed samples.

The ionizing radiation used to create the floor, or the cured elastomerzone between the substrate and the unirradiated elastomer caneffectively be used to control the thickness of the floor or anchoringzone. Ionizing radiation attenuates in the composition in directproportion to its energy. The thickness of the zone varies almostexactly linearly with the intensity of the radiation used (given aconstant time exposure). For example, with this Example, a 200 Kvexposure produces an 8 mil (0.250 mm) floor, 210 Kv a 10 mil (.254 mm)floor, and 240 Kv a 14 mil (0.332 mm) floor.

Another unique aspect of the present invention is the recyclable natureof the developmentally removed composition and the development sheet.After removal of the unexposed composition from the photosensitivearticle, the absorbent layer with the composition therein can be furthertreated to remove the absorbed or entrapped composition and reuse boththe recovered composition and the absorbent layer. The composition canbe removed simply by blowing heated air (above the softening temperatureof the composition) through the absorbent layer so that the compositionsoftens enough to be forced out of the absorbent layer into a receivingtray or vessel. The hot air may be blown by a slot vent over the widthof the absorbent layer or the layer may be wrapped or carried about adrum with heated air directed out of the drum and through the layer. Theless viscous the composition is made by heating, the less force isneeded to remove it. The combination of heating and forced air can ejectthe composition in the form of droplets which are directed towards acooling bath (e.g., water). They will then form pellets which can bereheated/redissolved and be coated out again onto a printing platesurface. The recycled pellets would probably be combined with virginformulation to assure that any property changes occurring duringdevelopment and recycling were minimzed.

EXAMPLE 3A Curable Elastomer Composition from N,N-Bis)2-hydroxyethyl),N'-(methacryloxy-ethyl)urea unsaturated diol and poly-1,2-(butyleneoxide) diol

A radiation curable polyurethane elastomeric composition was prepared ina twin-screw extrusion reactor as follows:

I. A polyol mixture was prepared of the following components which werethoroughly mixed in a feed tank until homogenious:

A. 36.9 parts (0.0369 moles) of a 1000 molecular weightpoly-1,2-(butylene oxide) diol (Dow Chemical Co.);

B. 7.8 parts (0.0867 mole) of 1,4-butane-diol (GAF Chemical Co.)

C. 4.4 parts of a mixture of 2.97 parts (0.0115 moles ofN,N-Bis(2-hydroxyethyl), N'-(methacryloxy ethyl)urea unsaturated dioland 1.43 parts (0.0014 mole) of the same polyol as in charge A;

D 0.5 parts of α,αdiethoxy acetophenone (Irgacure-651, Ciba Geigy Co.)

E. 0.015 part methylene blue

F. 0.01 part ferric chloride

G. 0.04 part dibutyl tin dilaurate

II. A precision flow metering system was utilized to meter the abovepolyol stream into the inlet port of a 34 mm twin-screw counter rotatingextruder (Leistritz Co.) at a ratio of 57.13 parts by weight of polyolstream to 42.87 parts by weight 4,4'-bis-(isocyanato cyclohexyl) methane(Desmodur™ W-Mobay Chemical Co.). At this mass ratio, there was a slightequivalency excess of isocyanat emoieties in the feed stream relative tohydroxyl moisties. The reaction temperature of the polymerizing mass wasmaintained in the range of 150° to 150° C. as polymerization occurred inthe extruder. Upon exitting the extruder, the fully reacted curableelastomer composition was segregated into pellets of diameter ofapproximately 0.3 cm which were collected for further processing. Thecompleteness of the polymerization reaction was determined by monitoringthe infrared spectrum of a cast film of the curable elastomer productand determining the absorbance ratio of the --NCO absorption band (2250cm⁻¹) to the --CH₂ -- absorption band (2950cm⁻¹), which ratio proved tobe less than 0.2 indicating a complete reaction with only a slightexcess of --NCO groups remaining. The melt index of the curableelastomer composition was monitered at a temperature of 160° C. using aload of 1100 grams on the heated chamber of the Extrusion Plastometerand found to be in the range between 10 and 20 grams per 10 minuteinterval.

The fully reacted curable polyurethane elastomeric Composition 3Aconsisted of the following mole ratios of constituents:

    ______________________________________                                        Component               Moles                                                 ______________________________________                                        4,4'-bis-(isocyanato cyclohexyl)methane                                                               3.703                                                 1,4-butane-diol         2.262                                                 N,N-Bis(2-hydroxyethyl),N'-                                                                           0.299                                                 (methacryloxy ethyl)urea                                                      poly 1,2-(butylene oxide)-diol                                                                        1.000                                                 ______________________________________                                    

By calculation, Composition 3A possesses one unsaturated crosslinkingsite per every 7,530 molecular weight units, and 55.6% by weight hardsegment units.

EXAMPLE 4A Curable Elastomer Composition from N,N-Bis(2-hydroxyethyl),N'-(methacryloxy ethyl)urea unsaturated diol,poly-1,2-(butylene-oxide)-diol and poly-(epichloridrin)-diol

A radiation curable polyurethane elastomeric composition was prepared ina twin-screw extrusion reactor as follows:

I. A polyol mixture was prepared of the following components which werethoroughly mixed in a feed tank until homogenious:

A. 13.0 parts (0.0130 moles) of a 1000 molecular weight poly1,2-(butylene oxide)-diol (Dow Chemical Co.);

B. 13.0 parts 90.0059 moles) of a 2200 molecular weightpoly-epichlorohydrin-diol) (3M Co.);

C. 6.4 parts (0.0711 mole) of 1,4-butane diol (GAF Chemical Co.)

D. 3.14 parts of a mixture of 2.13 parts (0.008 moles ofN,N-Bis(2-hydroxyethyl), N'-(methacryloxy ethyl) urea unsaturated dioland 1.01 parts (0.0010 mole) of the same polyol as in charge A;

E. 0.5 parts of α,αdiethoxy acetophenone (Irgacure-651, Ciba-Geigy Co.);

F. 0.01 part methylene blue

G. 0.007 part ferric chloride

H. 0.03 part dibutyl tin dilaurate

II. A precision flow metering system was utilized to meter the abovepolyol stream into the inlet port of a 34 mm twin-screw counter rotatingextruder (Leistritz Co.) at a ratio of 57.76 parts by weight of polyolstream to 42.24 parts by weight 4,4'-bis (isocyanato cyclohexyl) methane(Desmodur™ W-Mobay Chemical Co.). At this mass ratio, there was a slightequivalency excess of isocyanate moieties in the feed stream relative tohydroxyl moieties. The reaction temperature of the polymerizing mass wasmaintained in the range of 150° to 170° C. as polymerization occurred inthe extruder. Upon exiting from the extruder, the fully reacted curableelastomer composition was segregated into pellets of diameter ofapproximately 0.3 cm which were collected for further processing. Thecompleteness of the polymerization reaction was determined by monitoringthe infrared spectrum of a cast film of the curable elastomer productand determining the absorbance ratio of the --NCO absorption band (2250cm⁻¹) to the --CH₂ -- absorption band 2950 cm⁻¹), which ratio proved tobe less than 0.2 indicating a complete reaction with only a slightexcess of --NCO groups remaining. The melt index of the curableelastomer composition was monitered at a temperature of 160° C. usingaload of 1100 grams on the heated chamber of the Extrusion Plastometerand found to be in the range between 25 and 35 grams per 10 minuteinterval.

The fully reacted curable polyurethane elastomeric Composition 4Aconsisted of the following mole ratios of constituents:

    ______________________________________                                        Component                Moles                                                ______________________________________                                        4,4'-bis-(isocyanato cyclohexyl)methane                                                                5.246                                                1,4-butane diol          3.760                                                N,N-Bis(2-hydroxyethyl), N'-(methacryloxy                                                              0.432                                                ethyl)urea                                                                    poly 1,2-(butylene oxide)-diol                                                                         0.6875                                               poly(epichlorohydrin)diol                                                                              0.3125                                               ______________________________________                                    

By calculation, Composition 4A possesses one unsaturated crosslinkingsite per every 7,401 molecular weight units, and 57% by weight hardsegment units.

EXAMPLE 5B

A flexographic printing plate was prepared using the absorption meltprocessing method utilizing a DuPont PLS flexographic plate product. Thesheet flexo plate making material was removed from the shipping cartonand cut into a 30 cm×30 cm square section. Backside irradiation wasaccomplished in a Kelleight Model #210 plate exposure/processor unit fora period of 20 seconds. Following this step, the protective cover filmwas removed and a graphic arts test negative film was applied to thesurface of the plate with the cover film attached to the exposure unitthen applied to this "sandwich." After a 2 minute period during whichair was exhausted between the exposure negative and the curableelastomer surface, an imagewise exposure was accomplished for a periodof 10 minutes. After bleeding of the vacuum and removal of the exposurenegative, the plate was developed by removal of the unexposed anduncured areas of the elastomeric layer using the absorption method inthe same manner as described in Example 2B. A total of (6) passes of theplate material through the heated laminator rolls was required to removeat least 90% of the unexposed and uncured portions of the elastomerlayer. Fresh non-woven CEREX™ web material was utilized for each pass soas to allow for maximum absorption to occur. Excellent definition of therelief areas of the resulting printing plate sample was apparent asdetermined by visual inspection using a hand-held microscope of 10Xmagnifying power. The total caliper of the printing plate was 1.7 mmwith a relief depth measured at 1.0 mm for the raised areas of theplate.

EXAMPLE 6B

A flexographic printing plate was prepared utilizing the absorptionprocessing method using a BASF Nyloflex flexographic plate product.After removal of the product from the packaging carton, a section wascut having dimensions of 10 cm×20 cm×2.84 mm. The backside and imagewiseexposure were exactly as described in Example 5B. The removal of theunexposed and uncured areas of the plate was accomplished in a mannersimilar to Example 5B also. After (6) passes through the heatedlaminator rolls using fresh CEREX™ non-woven web for each pass, aflexographic printing plate having a relief of 1.27 mm was produced.Half-tone dot definition was excellent as determined by visualinspection using a hand-held microscope of 10X magnifying power.

The novel radiation curable polyurethane elastomers used in the practiceof the present invention may be described in a number of ways, inaddition to the chemical characterizations given above. The elastomercontains two segments with different physical properties. There is thehard segment made up of the diisocyanate and short chain di-freehydrogen chain extenders. The remaining segment produced by theremaining reactants. It has been found that a broad preferred range ofhard segments of the elastomer is from 25-60% of the total weight of theelastomer. A more preferred proportion for the hard segment is 30 to 50%by total weight of the elastomer. These ranges effectively includeproportions of the individual components as follows:

A) 10-40% by weight of the at least one diisocyanate,

B) 0.5-20% by weight of said first di-free hydrogen containing chainextending agent containing at least one ethylenically unsaturated groupper molecule (preferably as a pendant group) (preferably 0.75 to 12%),

C) 0.5-20% by weight of said second di-free-hydrogen containing chainextending group (which may be free of polymerizable ethylenicunsaturation) (preferably 0.75 to 12%), and

D) 10-60% by weight of said polyol.

Preferred elastomers of the present invention having the requisitephysical properties described above (e.g., molecular weight, melttransition temperature, and melt index) would be composed of unitsderived from the following percentages of reactants (which wouldapproximate the weight percentage of polymer units derived from thosereactants):

A) 15 to 35% by weight of the diisocyanate,

B) 1 to 10% by weight of said first chain extending agent,

C) 1 to 15% by weight of said second chain extending agent, and

D) 20 to 60% by weight of said polyol.

I claim:
 1. A process for producing a flexographic printing platecomprising providing a relief imageable element comprising a flexiblesubstrate having on at least one surface thereof a radiation hardenablecomposition in a thickness of at least 0.3 mm, imagewise irradiatingsaid composition to harden the composition in irradiated areas,contacting said imagewise irradiated layer with an absorbent layer whichcan absorb unirradiated composition when it has been heated between 40°and 200° C., heating said composition layer so that it is at atemperature between 40° and 200° C. while in contact with said absorbentlayer, said temperature being sufficiently high to enable saidcomposition in unirradiated areas to flow into said absorbent layer,allowing at least 75% of composition from unirradiated areas in contactwith said absorbent layer to be absorbed by said absorbent layer, andremoving said absorbent layer and said at least 75% of composition fromunirradiated areas from said flexible substrate wherein after saidabsorbent layer is removed from said flexible substrate, the absorbentlayer is heated to soften and remove at least some of said compositionfrom said absorbent layer.
 2. The process of claim 1 wherein saidcomposition comprises a polymeric elastomer.
 3. The process of claim 1wherein gas or liquid flow is used in combination with heat to remove atleast some of said composition from said absorbent layer.
 4. The processof claim 3 wherein said composition comprises a polymeric elastomer. 5.A process for producing a flexographic printing plate comprisingproviding a relief imageable element comprising a flexible substratehaving on at least one surface thereof a radiation hardenablecomposition in a thickness of at least 0.3 mm, imagewise irradiatingsaid composition to harden the composition in irradiated areas,contacting said image irradiated layer with an absorbent layer which canabsorb unirradiated composition when it has been heated between 40° and200° C., heating said composition layer so that it is at a temperaturebetween 40° and 200° C. while in contact with said absorbent layer, saidtemperature being sufficiently high to enable said composition inunirradiated areas to flow into said absorbent layer, allowing at least75% of composition from unirradiated areas in contact with saidabsorbent layer to be absorbed by said absorbent layer, and removingsaid absorbent layer and said at least 75% of composition fromunirradiated areas from said flexible substrate wherein said compositioncomprises an elastomer which displays less than 2% swell in deionizedwater at 20° C. for 24 hours, and further comprises from 0.3 to 10% byweight of a free radical photoinitiator, and wherein after saidabsorbent layer is removed from said flexible substrate, the absorbentlayer is heated to soften and remove at least some of said compositionfrom said absorbent layer.
 6. The process of claim 2 wherein gas orliquid flow is used in combination with heat to remove at least some ofsaid composition from said absorbent layer.